Low pressure separation for light hydrocarbon recovery

ABSTRACT

Net horsepower required for recovering high purity ethylene is reduced by process gas compression of furnace effluent to only low pressures before entering recovery facilities. The compressor discharge undergoes chilling in heat exchangers or pumparound towers designed for small pressure drops. Hydrocarbons condensed during chilling are pumped to higher pressures for fractionation.

This invention relates to the separation of light ends fractionsobtained in petroleum processing which may contain hydrogen, methane,ethylene, ethane, propylene, propane and C₄ 's and heavier, as well assmall amounts of acetylene and carbon monoxide. Such fractions may bederived from various petroleum refinery sources and from pyrolysis ofhydrocarbons including catalytic cracking; and thermal cracking such aspartial combustion cracking, in particular cracking in the presence ofsteam.

Steam cracking is a well-known process and is described in U.S. Pat. No.3,641,190 and British Pat. No. 1,077,918, the teachings of which arehereby incorporated by reference. In commercial practice, steam crackingis carried out by passing a hydrocarbon feed mixed with 20-92 mol.%steam through metal pyrolysis tubes located in a fuel fired furnace toraise the feed to cracking temperatures, e.g., about 1200° to 1800° F.and to supply the endothermic heat of reaction for the production ofproducts including unsaturated light hydrocarbons, particularly C₂ -C₄olefins and diolefins, especially ethylene, useful as chemicals andchemical intermediates.

Ethylene in particular is in great demand for the synthesis of organicchemicals such as glycols, alcohols, plastics and solvents. Cracking ofhydrocarbons which may range from gases up to liquids such as naphthaand gas oil, in the presence of steam has been widely used for itsproduction, in fact such plants are termed ethylene plants. Since theresolution of the light ends fraction into its constituents, in thecryogenic section or cold end of the steam cracking plant, consumeslarge amounts of energy owing to the compression/refrigerationrequirements when such low boiling gaseous materials are separated bydistillation, there is a need for a process which would achieve savingsin energy. Nonetheless, separation methods in common use have remainedrelatively static for many years.

BACKGROUND OF THE INVENTION

Conventional recovery facilities for ethylene plants via fractionationgenerally involve compression of cracked feed gas to high pressures,400-600 psia, e.g., about 550 psia, followed by extensive chilling andfractionation for products separation. The normal sequence offractionation is demethanization, followed by deethanization,depropanization and debutanization. For purposes of comparison suchprocess will be termed herein "Base Case". It may be noted that suchprocess requires the entire gaseous effluent of cracking to becompressed to, e.g., 550 psia, of which about one third is tail gas,viz., H₂ +methane. However, tail gas is not required to be compressed tothat level--because for subsequent use it is generally let down to muchlower pressure--and the energy used up in doing so cannot all berecovered. The present invention aims to avoid high compression of tailgas.

A high pressure process is discussed by J. R. Fair et al. in Chem. Eng.Prog., 54 (No. 12) 39-47, December 1953. Particular emphasis is placedon the separation of the gas into two fractions: one comprising methaneand lighter materials and the other comprising ethylene and heaviermaterials, viz., demethanization which is said to be the major step. Themost popular level of feed gas compression in the United States isstated to be 450-600 psia. In this article, feed gas, from which themajority of C₅ and heavier components have been removed, is compressedto this level. The authors note that the principal energy requirementsfor demethanization are feed gas compression and refrigeration. It isalso mentioned that pre-removal of unsaturated C₄ and heavier materialsmay be desirable in some instances. U.S. Pat. No. 2,938,934 discloses amethod wherein, in an initial depropanizer, C₄ and heavier materials areremoved as bottoms. The overhead gas is then compressed to 450 psia andpassed to a demethanizer; the deethanizer follows the demethanizer.Therefore, high compression of tail gas is not avoided. In U.S. Pat. No.3,729,944 a cracked gas is subjected to staged compression withinterstage cooling and a gas-liquid mixture therefrom is introduced intoa first fractionation zone at 177 psia. The C₄₊ materials are removed asbottoms and the overhead is compressed to 335 psia and processed forfurther fractionation/separation. Here also there is compression of tailgas to high levels.

Other literature of general interest includes:

• Pratt and Foskett, Trans. Amer. Inst. Chem. Engrs. 42, 149 (1946).

• W. K. Lam et al., Oil and Gas Jl. 111, May 18, 1970.

SUMMARY OF THE INVENTION

According to the invention, a feed gaseous mixture containing lighthydrocarbons, e.g., in the range of C₁ to C₅₊, and which may alsocontain hydrogen, is separated by compressing the feed gas to a pressurein the range of 40 to 125 psia, preferably 50 to 100 psia, passing thegas in series through a series or cascade of refrigeration zonesmaintained at temperatures that progressively decrease in the series, tocondense in each zone a liquid portion of increasing volatility in theseries, passing or pumping, as need be, the liquid portion of each zoneto a connected, respective fractionation tower maintained at suitabletemperature and pressure conditions for effecting fractionation thereofand carrying out said fractionation, thereby achieving separation of thefeed gas into fractions.

Preferably, the feed gas is the light ends from the thermal cracking,especially steam cracking, of hydrocarbons, particularly liquidhydrocarbons such as naphtha and gas oil. The invention is alsoapplicable to feed gas containing essentially C₁ to C₄ hydrocarbons withor without hydrogen.

Although much of the energy not expended in compression is shifted toincreased pumping and refrigeration requirements, it has surprisinglybeen found that, within the ranges of feed gas inlet pressure givenabove, there is a net reduction in energy consumption. In the separationof light ends from a pyrolysis process, this reduction is maximum forliquid hydrocarbon cracking feeds and some reduction can be obtained formixed liquid/gas hydrocarbon cracking feeds (such as when recoveredethane is recycled) so that it is desirable that at least some portionof the cracking feed be liquid. The method is capable of producing highpurity products, e.g., ethylene, propylene, etc.

In the subject process of fractional liquefaction or condensation,energy conservation is achieved partly via lowered gas compressionrequirements by virtue of the fact that:

(1) compression of total feed gas is only to relatively low pressurevalues, as contrasted with Base Case.

(2) separation is by refrigeration/condensation of highest-to-lowestmolecular weight fractions to leave a tail gas without the necessity ofcompression thereof.

(3) then, the refrigerated, condensed fractions are passed or pumped, asmay be necessary, to individual fractionation towers for rectification,still without compressing tail gas.

It follows, therefore, that tail gas is never compressed to highpressures as practiced in the prior art.

Pumping liquid to increase pressure uses less energy than compressinggas. Also, coldest refrigeration (which requires the most energy toproduce, of the various refrigeration levels used) is applied to thesmallest mass since this is only what is left after removing the morereadily condensible materials--this also makes it possible to saveenergy.

In summary, the energy savings are mainly attributed to two factors:

(1) tail gas is not compressed to pressure higher than necessary.

(2) hydrocarbons are pumped rather than compressed to high pressures foreconomic fractionation.

As contrasted with conventional operation in which the feed frompyrolysis is compressed in several stages with interstage cooling, tohighest pressures, e.g., about 550 psia, then sent through a cascade ofdistillation towers at progressively lower pressures, the presentinvention is characterized by sharply lower feed gas compression outletpressure--in which sense it is termed a low pressure separationprocess--and by the reverse process sequence, viz., removing thehighest-to-lowest molecular weight fractions in the sequence so thatdemethanization occurs last.

In a preferred embodiment, a gaseous mixture which contains lightolefinic constituents and includes C₅₊ 's, viz., it may contain smallamounts of hydrocarbons vicinal to C₅ such as C₆ -C₉, down to methaneand hydrogen, is subjected to cooling in a first refrigeration zone tocondense a portion concentrated in C₄ -C₅₊ which is removed; theoverhead from said first zone is subjected to further cooling to lowertemperatures in a second refrigeration zone to condense a portionconcentrated in C₃ -C₄ which is removed; the overhead from said secondzone is subjected to further cooling to lower temperatures in a thirdrefrigeration zone to condense a portion concentrated in C₂ -C₃ which isremoved; and the overhead from said third zone is subjected to furthercooling to lower temperatures in a fourth refrigeration zone to condensea portion concentrated in C₁ -C₂ and leave a tail gas. The condensed C₁-C₂ fraction is pumped to a demethanizer wherein at increased pressureand suitable temperature a purified liquid C₂ fraction is separated frommethane. It can be seen, therefore, that the overhead comprising tailgas of said fourth refrigeration zone is never subjected to highpressure. The aforementioned other condensates are likewise crudefractions since a clean separation to achieve the high purity desirablefor commercial use, is not made in the refrigeration zones. Therefore,to attain the wanted purity, each is suitably passed or pumped to aconnected, respective fractionation tower in which it is rectified,preferably with addition of overhead vapor and/or liquid from the nextpreceding fractionation tower (except in the case of the first tower).Thus, liquids condensed in the chilling process are pumped to higherpressures for fractionation.

BRIEF DESCRIPTION OF THE DRAWING

The drawing illustrates the process sequence and suitable equipmenttherefor, of a preferred embodiment of the invention. Like numbers areused to designate like parts.

DETAILED DESCRIPTION

In a liquids steam cracker with ethane being recycled to the crackingfurnace, a lights end fraction is obtained from the primaryfractionator, having the typical mol.% composition shown in Table I,which is not to be considered as limiting the invention:

                  TABLE I                                                         ______________________________________                                        Component         Mole Percent                                                ______________________________________                                        H.sub.2           12.3                                                        Methane           21.5                                                        Ethylene          25.0                                                        Acetylene         0.30                                                        Ethane            7.94                                                        Propylene         11.0                                                        Propane           0.52                                                        Methyl acetylene  0.41                                                        Butadiene         2.46                                                        Isobutylene       2.48                                                        Butene-1          0.74                                                        Cis-butene-2      0.68                                                        Pentene-1         0.74                                                        Pentane           0.83                                                        Isopentane        0.68                                                        2-methylpentane   0.55                                                        Cis-hexene-2      0.35                                                        Benzene           1.68                                                        Toluene           1.26                                                        Metaxylene        0.85                                                        1,2,3-trimethyl benzene                                                                         0.32                                                        H.sub.2 O         7.80                                                        Total             100.0                                                       ______________________________________                                    

It will be understood that the composition can vary depending on thespecific liquid hydrocarbon feed chosen and on the parameters selectedfor cracking temperature, residence time and hydrocarbon partialpressure in the pyrolysis zone which affect selectivity to ethylene.

Further, the following detailed description refers to a particularembodiment which is to be considered illustrative and not limiting.

With reference to the drawing, suitable apparatus comprises a processgas compressor, a refrigeration train and a fractionation train.

In general, the technique of multi-stage gas compression and the removalof the compression heat obtained in each compression stage is known inthe art--see for example U.S. Pat. No. 3,947,146. However, in thepresent process fewer compression stages are needed, in the preferredembodiment here described only two.

The compression effluent then undergoes extensive refrigeration inpumparound towers which minimize pressure drop. Alternatively,conventional heat exchangers may be used in place of pumparound towersfor chilling purposes but the pressure drop during chilling will beslightly higher.

Referring to the drawing, feed gas of the composition described above at21 psia is introduced by line 3 into first stage compressor 1 where, inthis illustration, it is compressed to 38 psia, then passed by line 5through heat exchanger 7 cooled by cooling water (CW), then through line9 into drum 11 where vapor and liquid are separated. The vapor is thenpassed by line 13 into second stage compressor 2 where, in thisillustration, it is compressed to 67.5 psia, then passed by line 15 toheat exchanger 17, then through line 19 into drum 21 where vapor andliquid are separated. Suitably the vapor is then passed by line 23 tocaustic and glycol treatment facilities 25 where it is treated to removetrace amounts of acidic gases such as CO₂, H₂ S as well as water, in amanner known per se. The treated gas is then introduced via line 27 intothe bottom of the first of the pumparound refrigeration towers, T-1. T-1also receives, at the top, via line 4, a liquid C₄ stream taken from thebottom of the depropanizer then cooled by cooling water in heatexchanger 6 and chilled in chilling unit 8 and at 2° F. and 60 psiapassed into T-1. Valve 10 may be used to control flow of this stream.The liquid C₄ stream introduced by line 4 and the gaseous effluentintroduced by line 27 flow countercurrent to one another andequilibration between lighter gas and heavier liquefying phases takesplace in refrigeration tower T-1. This is typical of operation in T-2,T-3 and T-4 (except that liquid introduced at the top of the tower is ofdecreasing carbon number, e.g., C₃, C₂ and C₁).

A bottoms stream is withdrawn from T-1 via line 29, pumped by means ofpump 31 through heat exchanger 33 wherein it is cooled, and re-entersT-1. A portion is withdrawn by line 49. A mid-section stream iswithdrawn via line 35, pumped through heat exchanger 37 wherein it iscooled, and re-enters T-1. A portion is withdrawn by line 47. An upperstream is withdrawn via line 39, pumped through heat exchanger 41wherein it is cooled, and re-enters T-1. A portion is withdrawn by line45. A top stream is withdrawn by line 43. For simplicity, not all pumpsare shown. This repeated cooling of withdrawn streams causes atemperature gradient to be established in the column, the temperaturedecreasing up the column. This brings about a rough separation. Theheavier, more readily condensible materials in the zone become liquidand drop down, the lighter, more volatile materials concentrating in thegas which goes to the top. The same phenomena occur in refrigerationtowers T-2, T-3 and T-4. The temperatures of the refrigerants used inthe various heat exchangers, for this embodiment, are shown in thedrawing, and are also not be considered as limiting the invention.Refrigerants for the heat exchangers are selected from, e.g., liquidpropylene (for refrigerant levels of 53° down to -52° F.), ethylene (forlevels of -72° down to -152° F.) and methane (for levels of -167° downto -217° F.) to obtain the desired process temperatures. The overheadgas of T-1 is essentially devoid of C₅₊ materials and rich in lighter,viz., it is a C₄₋ fraction. The liquid streams taken out of T-1 containall of the C₅₊ materials and substantial amounts of C₄ 's and somelighter, viz., they are C₄ -C₅₊ concentrates. Valves 40 may be used tocontrol flow of the liquid streams.

Liquid sidestreams taken from T-1 via lines 43, 45 and 47 (supplementedby line 49) respectively at temperatures of -8° F., 28° F. and 60° F.,are passed into the debutanizer. Since the pressure, 59 psia, in T-1 ishigher than the pressure, 42 psia, in the debutanizer, pumping is notnecessary and the liquids flow to the debutanizer. In passing, thestreams are used for process duty, i.e., for cooling other planteffluents so that they themselves become warmer. This is likewise trueof other streams for which process duty is indicated in the drawing.Typically there is a mixture of gas and liquid going into thefractionating towers which are run warmer anyway. Recovering processduty at these cold temperatures reduces refrigerant compressionrequirements. Low pressure steam (L.P. Stm) consumption in thefractionating tower reboilers is also reduced.

The debutanizer is run at a top temperature of 64° F. and a bottomtemperature of 195° F. It separates a liquid bottoms C₅₊ product whichis removed and passes overhead a C₄₋ fraction. Since the debutanizer isonly at a pressure of 42 psia and the overhead is to be passed partlyinto refrigeration tower T-2 at 54 psia, the vapor overhead must becompressed in the debutanizer overhead vapor product compressor to 65psia to effect this purpose. Therefore, it is passed via line 51 intoheat exchanger 53 where it is cooled to 32° F., then passed to drum 54where vapor and liquid are separated. The vapor portion is then passedvia line 55 into the compressor where it is compressed to 65 psia,passed via line 57 into heat exchanger 59 where it is cooled to -2° F.,followed by a separation of vapor and liquid in drum 60. The vaporportion from drum 60 is passed via lines 61 and 67, and at a pressure of56 psia and a temperature of -14° F. is introduced into the bottom oftower T-2. A portion of the liquid from drum 54 is returned by line 56as reflux to the debutanizer while the remaining liquid is passed byline 58 to join the liquid portion from drum 60 in line 62.

The combined stream is pumped to 215 psia in pump 64 and passed to thedepropanizer by line 66 at a temperature of 20° F. and a pressure of 213psia after giving up some refrigeration for process duty.

The overhead from tower T-1 is passed by line 63 into a guard drier toremove traces of water and then by line 65 to join the effluent in line61 and pass into the bottom of tower T-2 by line 67. T-2 also receives,at the top, via line 69, a highly concentrated liquid C₃ stream, labeledA, taken from the bottom of the deethanizer, cooled by cooling water inheat exchanger 70 and chilled in chilling unit 72, and, at -58° F. and55 psia, introduced into T-2.

T-2, run at lower temperatures than T-1, similarly separates liquidsidestreams from a gaseous overhead. Liquid sidestreams taken from T-2via lines 43', 45' and 47', respectively, at temperatures of -69° F.,-45° F. and -30° F. and a liquid bottoms stream taken via line 49' arepumped by pumps 71, 73, 75 and 77 respectively into the depropanizer.This is necessitated because the pressure, 54 psia, in T-2 is lower thanthe pressure, 205 psia, in the depropanizer. The liquids are pumped tohigh pressure before refrigeration is recovered as process duty. Valves40' may be used to control flow of the liquid streams. The overhead gasof T-2 is essentially devoid of C₄₊ materials and rich in lighter, viz.,it is a C₃ -fraction. The liquid streams taken out of T-2 areconcentrated in C₃ -C₄ 's as is also the overhead stream from thedebutanizer passed into the depropanizer.

The depropanizer is run at a top temperature of 84° F. and a bottomtemperature of 195° F. It separates a liquid buttoms C₄ product which isremoved, a portion being sent to T-1 as aforesaid, and passes overhead avapor C₃ -fraction, which is condensed in heat exchanger 78, pumped to510 psia in pump 76 and passed into the deethanizer by line 74. This isnecessitated because the pressure, 205 psia, in the depropanizer islower than the pressure, 490 psia, in the deethanizer. The overhead fromtower T-2 is passed by line 79 into the bottom of tower T-3. T-3 alsoreceives, at the top, via line 81 a highly purified liquid C₂ stream,labeled B, taken from the bottom of the demethanizer, chilled inchilling unit 84 and, at -113° F. and 53 psia, introduced into T-3.

T-3, run at lower temperatures than T₂, similarly separates liquidsidestreams, in this case at temperatures of -127° F., -105° F. and -90°F., which are pumped from a pressure of 52 psia up to high pressure tointroduce them into the deethanizer which is maintained at 490 psia. Theexplanation given with regard to T-2 sufficiently explains the operationof T-3 and T-4. The overhead gas of T-3 is essentially devoid of C₃₊material and rich in lighter, viz., it is a C₂₋ fraction. The liquidstreams taken out of T-3 are highly concentrated in C₂ -C₃ 's as is alsothe overhead stream from the depropanizer passed into the deethanizer.

The deethanizer is run at a top temperature of 29° F. and a bottomstemperature of 171° F. It separates a liquid bottoms C₃ product which isremoved, a portion being sent to T-2 is aforesaid, and sends overhead avapor C₂ -fraction which is passed into the demethanizer via line 80after being chilled in heat exchanger 82 to -5° and in heat exchanger 86to -30° F. The overhead from tower T-3 is passed by line 83 into thebottom of tower T-4. T-4 also receives, at the top, via line 85 a97+mole% methane stream, labeled C, taken from the top of thedemethanizer, cooled in heat exchanger 88 to -137° F. and chilled inchilling unit 92, then at -228° F. and 51 psia, introduced into T-4.

The C₃ bottoms product of the deethanizer, is sent to propylene recoveryfacilities which suitably may include a propylene hydrogenation unit(propylene hydrofiner) and fractionation tower (propylene rerun tower)as shown in the drawing. Chemical grade propylene product (93+mole%) isrecovered as overhead from the fractionation tower. A C₃ splitter mayfollow to produce polymer grade propylene (99+mole%).

T-4, run at the lowest temperatures of all the refrigeration towers,similarly separates liquid sidestreams, in this case at temperatures of-220° F., -180° F., -160° F., and 145° F., which are pumped from apressure of 50 psia up to high pressure to introduce them into thedemethanizer which is maintained at 440 psia. The overhead tail gas ofT-4 at 50 psia and -241° F. is recovered via line 87 and, after givingup its refrigeration for process duty, may be used as fuel gas or forother purposes. In this illustration the composition of the tail gas inline 87 is as follows in Table II:

                  TABLE II                                                        ______________________________________                                        Component     Mole Percent                                                    ______________________________________                                        H.sub.2       5.19                                                            Methane       94.34                                                           Ethylene      0.47                                                            Acetylene     <0.01                                                           Ethane        <0.0002                                                         Total         100.00                                                          ______________________________________                                    

The liquid streams 89,90 taken out of T-4 and the overhead stream 80from the deethanizer passed into the demethanizer are essentiallycomprised of C₂ 's and C₁ although varying considerably in percentagecomposition as shown in Table III below:

                  TABLE III                                                       ______________________________________                                                Mole Percent                                                          Component Stream 80   Stream 89  Stream 90                                    ______________________________________                                        H.sub.2   0.089       0.064      0.075                                        Methane   5.64        11.81      43.73                                        Ethylene  64.14       70.34      53.16                                        Acetylene 1.09        0.65       0.11                                         Ethane    28.92       17.14      2.92                                         C.sub.3   0.12                                                                Total     100.0       100.0      100.0                                        ______________________________________                                    

The demethanizer is run at a top temperature of -123° F. and a bottomstemperature of 18° F. It separates a liquid bottoms pure C₂ fraction ofthe composition shown in Table IV:

                  TABLE IV                                                        ______________________________________                                        Component     Mole Percent                                                    ______________________________________                                        Methane       0.0076                                                          Ethylene      75.23                                                           Acetylene     0.90                                                            Ethane        23.81                                                           Propylene     0.048                                                           Propane       0.0049                                                          Total         100.0                                                           ______________________________________                                    

and passes overhead a concentrated methane vapor stream C which is sentto tower T-4 as aforesaid.

The C₂ bottoms product of the demethanizer is sent to a C₂ splitter,suitably after being treated with hydrogen in an acetylene converter toremove traces thereof. The C₂ splitter fractionates the C₂ feed into anoverhead pure (99.95+mole%) ethylene product and a liquid bottoms pure(99+mole%) ethane product.

The saving in BHP (brake horsepower) is shown in Table V by thecomparison of Base Case with the present invention for two feed gasinlet pressures, 67.5 psia (Case 1) and 117.5 psia (Case 2):

                  TABLE V                                                         ______________________________________                                                           Present Invention                                                   Base Case   Case 1  Case 2                                           ______________________________________                                        Total BHP  92150         85900   91550                                        Δ BHP                                                                              --            (6250)  (600)                                        Δ% of BHP                                                                          --            (6.78)  (0.65)                                       ______________________________________                                    

Thus, it can be seen that for Case 1 a saving of 6.78% BHP is achievedover Base Case. Case 2 still uses somewhat less horsepower than BaseCase. Therefore, Table V shows that lower feed gas inlet pressure in thelight ends recovery facilities of a steam cracker reduces operatingcosts.

What is claimed is:
 1. A process for separating a feed gaseous mixturecontaining light hydrocarbons comprising C₁ to C₅₊ hydrocarbons with orwithout hydrogen, into fractions, which comprises compressing the feedgas to a compressor discharge pressure in the range of 40 to 125 psiaprior to passing it to a recovery operation, passing the compressed feedgas in series through a sequence of refrigeration zones maintained attemperatures that progressively decrease from first to last in thesequence to condense in each zone a liquid portion of increasingvolatility from first to last in the sequence, passing or pumping asrequired the liquid portion of each zone to a connected, respectivefractionation tower maintained at suitable temperature and pressure toeffect fractionation thereof and carrying out said fractionation,thereby achieving separation of the feed gas into fractions, to obtain asaving in energy as compared with carrying out said process employing acompressor discharge pressure above said range.
 2. The process accordingto claim 1 in which the feed gas is compressed to a pressure in therange of 50 to 100 psia.
 3. The process according to claim 2 in whichthe feed gas contains light olefinic constituents and comprises C₅ 'sdown to methane and hydrogen.
 4. The process according to claim 3 whichcomprises in combination the steps of:(a) compressing the feed gas to apressure in the range of 50 to 100 psia; (b) maintaining a series ofrefrigeration zones at temperatures that progressively decrease fromfirst to last in the series; (c) maintaining a series of fractionationtowers, respectively connected to a respective one of the series ofrefrigeration zones; (d) passing the compressed feed gas to the firstrefrigeration zone; (e) condensing a portion of the feed gas in thefirst refrigeration zone, passing the liquid to a respectivefractionation tower maintained at suitable temperature and pressure toeffect fractionation thereof and repeating a sequence of passing theresidual gas from a refrigeration zone to the next succeedingrefrigeration zone, condensing and fractionation as many times asdesired; and (f) recovering a tail gas from the last refrigeration zone.5. The process according to claim 4 in which the last of said series offractionation towers is a demethanizer.
 6. The process according toclaim 5 in which said series of fractionation towers comprises in thisorder: debutanizer, depropanizer, deethanizer and demethanizer.
 7. Theprocess according to claim 6 in which each fractionation tower in saidseries except the first receives distillate from the next precedingfractionation tower.
 8. The process according to claim 7 in which afirst refrigeration zone receives a C₄ fraction from the depropanizer, asecond refrigeration zone receives a C₃ fraction from the deethanizer, athird refrigeration zone receives a C₂ fraction from the bottom of thedemethanizer and a fourth refrigeration zone receives a C₁ fraction fromthe overhead of the demethanizer; each of said fractions being passed incountercurrent contact with feed or portions of feed to be separated. 9.The process according to claim 2 in which the feed gaseous mixture isthe light ends from a thermal cracking process.
 10. A process forseparating a feed gaseous mixture containing light olefinic constituentsand comprising C₅₊ 's down to methane and hydrogen, into fractions,which comprises in combination the steps of:(a) compressing the feed gasto a compressor discharge pressure in the range of 50 to 100 psia priorto passing it to a recovery operation; (b) cooling the compressed feedgas in a first refrigeration zone wherein it is passed in countercurrentcontact with a C₄ stream taken from a depropanizer, to condense a C₄-C₅₊ concentrate and passing the same to a debutanizer in which it isfractionated with removal of a liquid C₅₊ fraction; (c) cooling theresidual gas from said first zone in a second refrigeration zone whereinit is passed in countercurrent contact with a C₃ stream taken from adeethanizer, to condense a C₃ -C₄ concentrate, pumping the same to ahigher pressure and passing the same to the depropanizer in which it isfractionated in the presence of distillate from the debutanizer; (d)cooling the residual gas from said second zone in a third refrigerationzone wherein it is passed in countercurrent contact with a C₂ streamtaken from a demethanizer, to condense a C₂ -C₃ concentrate, pumping thesame to a higher pressure and passing the same to the deethanizer inwhich it is fractionated in the presence of distillate from thedepropanizer; (e) cooling the residual gas from said third zone in afourth refrigeration zone wherein it is passed in countercurrent contactwith a predominantly methane stream taken from the demethanizer, tocondense a C₁ -C₂ concentrate and separating tail gas from said fourthzone; and (f) pumping the condensed portion from said fourth zone to ahigher pressure, passing it into the demethanizer and fractionating thesame in the presence of distillate from the deethanizer to obtain apurified C₂ bottoms fraction and a predominantly methane vapor fraction;to obtain a saving in energy as compared with carrying out said processemploying a compressor discharge pressure above said range.
 11. Theprocess according to claim 13 in which the refrigeration zones comprisepumparound towers.
 12. The process according to claim 8 or 10 in which aportion of the debutanizer overhead vapor, after compression to suitablepressure, is partially passed to the second refrigeration zone, theremainder of said portion being passed to the depropanizer.
 13. Aprocess for separating a feed gaseous mixture containing lighthydrocarbons comprising C₁ to C₅₊ hydrocarbons with or without hydrogen,into fractions, which comprises compressing the feed gas to a compressordischarge pressure in the range of 40 to 125 psia, prior to passing itto a recovery operation, passing the compressed feed gas in seriesthrough a sequence of refrigeration zones maintained at temperaturesthat progressively decrease from first to last in the sequence tocondense in each zone a liquid portion of increasing volatility fromfirst to last in the sequence, passing or pumping as required the liquidportion of each zone to a connected, respective fractionation towermaintained at suitable temperature and pressure to effect fractionationthereof and carrying out said fractionation, thereby achievingseparation of the feed gas into fractions; said feed gaseous mixturebeing the light ends from a steam cracking process wherein at least aportion of the cracking feed is liquid hydrocarbon; to obtain a savingin energy as compared with carrying out said process employing acompressor discharge pressure above said range.
 14. The processaccording to claim 13 in which tail gas is recovered from the lastrefrigeration zone in the sequence.